The present invention relates to an idle speed adjustment method for an internal combustion engine which has an idle speed feedback control system.
There is known a feedback control system for controlling the idle speed of an internal combustion engine by controlling the flow rate of air taken into the engine when the engine is in the idling or decelerating condition, that is, when a throttle valve disposed in an intake passage of the engine is at the idle position. In such an idle speed feedback control system, the flow rate of intake air is controlled by an air control valve disposed in a passage bypassing the throttle valve. The air control valve controls either the cross-sectional area or the opened time period of the bypass passage in response to a drive signal representing the difference between the detected actual rotational speed of the engine and a desired idle speed. Adjustment of the idle speed of an internal combustion engine having such a system can be carried out by turning an idle speed adjustment screw, which is disposed in a second bypass passage connected to the intake passage parallel to the aforementioned bypass passage.
In a conventional idle speed adjustment method, the idle speed is adjusted under a condition in which the air control valve is not energized, that is, the air control valve is fully closed. Under this condition, according to the conventional method, the idle speed adjustment screw is turned in or out so that the actual speed of the engine becomes equal to a speed, for example, 500 rpm, which is lower than a desired idle speed, for example, 700 rpm. However, this conventional adjustment method has the following problems:
(1) Since the characteristics of air control valves usually lack uniformity, even if these air control valves are driven by signals of uniform value, the flow rate of air passing through each of the air control valves is different. Therefore, if the air control valves are driven by a uniform value signal, a desired idle speed cannot be obtained in each engine. As a result, in engines whose idle speeds are adjusted according to the conventional adjustment method, it is almost impossible to set the maximum value, minimum value and initial value of the drive signal for energizing the air control valve to uniform values, respectively.
(2) Since the engine speed adjusted according to the conventional adjustment method is very low, the engine runs very roughly during adjustment. Thus, the adjustment of the idle speed is not easy and accurate adjustment of idle speed cannot be expected.
(3) In the engine whose idle speed is adjusted by using the conventional adjustment method, the flow rate of air passing through the second bypass passage to which the idle speed adjustment screw is attached is relatively great, compared to the flow rate of air passing through the air control valve at idling. Therefore, the range of the air flow rate controlled by the air control valve is restricted. As a result, the automatic compensation range with respect to the change in idle speed, particularly to the change in idle speed caused by reduction of friction loss of the engine, according to the idle speed feedback control system cannot be enlarged.